Photolithography is a type of lithographic process used in the manufacturing of semiconductor devices, integrated optics and photomasks. In a photolithography process, a light is used to transfer a geometric pattern from a photomask to a substrate such as a silicon wafer. A photoresist layer is first formed on the substrate. The substrate is baked to remove any solvent remained in the photoresist layer. The photoresist is then exposed through a photomask with a desired pattern to a source of actinic radiation such as ultraviolet (UV), extreme ultraviolet (EUV), electron beams, or X-rays. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution, usually an aqueous base solution, to form a pattern in the photoresist layer. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
Conventionally, there are two types of photoresist: positive resist and negative resist. They are characterized by a dissolution curve in which there is a single transition from a first dissolution rate to a second dissolution rate as the photoresist is exposed to increasing levels of actinic radiation. A positive resist is initially insoluble in the developer solution. After exposure, the exposed region of the resist becomes soluble in the developer solution. A negative resist behaves in the opposite manner. The negative resist is initially soluble in the developer solution. Exposure to radiation typically initiates a crosslinking reaction which causes the exposed region of the negative resist to become insoluble in the developer solution. For both positive and negative photoresists, the soluble areas of the photoresist essentially dissolve in the developer to become spaces, while the insoluble areas remain as lines of the photoresist in a subsequent development step.
As semiconductor ground rule gets smaller, the need for printing a large number of lines and spaces in a given area has increased dramatically. In response, the semiconductor industry has taken several efforts to increase the resolution of a photolithography process. For example, phase shift masks, shorter wavelength exposure tools, immersion lithography, higher numerical aperture exposure tools, and tools with selective illumination systems are continuing to be developed to improve the pattern density of integrated circuits. However, all these approaches mentioned above come with high cost. In addition, it has become ever more challenging to print small space features such as trenches and vias of small dimensions with precise image control using conventional positive or negative photoresist due to the poor optical image contrast of the masks used to create these features. Therefore, there is a need for a photoresist composition and a pattern forming method that can achieve improved resolution with relatively low cost and can print small features with precise image control, particularly spaces of small dimensions.